According to the differences in biochemical reaction and pigment production, Staphylococcus can be divided into three species, Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus saprophyticus. Among them, Staphylococcus aureus is mostly pathogenic, Staphylococcus epidermidis occasionally pathogenic and Staphylococcus saprophyticus generally is non-pathogenic. Staphylococcus aureus is a common gram-positive coccus that causes many serious infections in humans and animals. It is also one of the common pathogens in hospital infections. Staphylococcus easily becomes resistant to antibiotics, including various common antibiotics and novel antimicrobial agents. The emergence and widespread of methicillin resistant Staphylococcus aureus (MRSA) has brought unprecedented challenges to clinical treatment. In order to deal with the problem of resistance to antibiotics in Staphylococcus, it is necessary to extract staphylococcal genome DNA for PCR to detect whether the clinical strains are MRSA and so on. However, owing to the tough cell wall of Staphylococcus, the normal egg white lysozyme has no obvious lytic effect on it, while lysostaphin that has good lytic activity against staphylococcal cell wall is too expensive to be widely applied.
Phage lysin is a kind of cell wall hydrolase expressed in late stage after host bacteria being infected with dsDNA phage. The size of lysins is normally in the range of 25 kD-40 kD, they are usually composed of two independent functional domains in the structure, the N-terminal catalytic domain, and a C-terminal cell wall binding domain (CBD) that determines cell binding sites. The two domains are linked by a small fragment. Sequence analysis shows that the catalytic domain of the same type lysin is highly conserved while the cell binding domain is variable, which provides the possibility for constructing new chimeric lysins. Lysins are highly specific and can only specifically identify and kill specific species of bacteria. Moreover, the action site of lysins is very conservative and coevolution of specificity of phage and bacterial makes it difficult for host bacteria to become resistant to them. These characteristics of lysins provide theoretical feasibility for the control and treatment of drug-resistant bacteria in clinical practice. So far, there are some natural lysins and chimeric lysins that have effect on Staphylococcus aureus. These enzymes have been reported to be able to kill Staphylococcus aureus well in vivo and in vitro. However, most of these lysins are difficult to be solubly expressed, or the activity is not high or they cannot adapt to protein-rich environment (such as milk, etc.), and the pH range to keep them to be active is narrow, generally in pH 5-8. It is very importantly significant to look for lysins which are solubly expressed in high level and have high activity for development of new anti-staphylococcal medicine and control of staphylococcal infections in vitro, and for detection of Staphylococcus by lysing its cell wall.